1. Field of the Invention
The present invention relates to non-reciprocal circuit elements, and particularly, to a non-reciprocal circuit element, such as an isolator and a circulator, for use in microwave bands and to a communication device.
2. Description of the Related Art
Generally, non-reciprocal circuit elements, such as isolators and circulators, have a characteristic that allows a signal to be transmitted only in a predetermined direction but not in the opposite direction. For example, by using this characteristic, isolators can be used in transmitting circuits of mobile communication devices, such as automobile telephones and portable telephones.
Japanese Unexamined Patent Application Publication No. 2002-26615 (Patent Document 1) discloses a non-reciprocal circuit element in which a ferrite assembly, including a ferrite and center electrodes, and permanent magnets are disposed vertically on a rectangular plate-like circuit substrate. The ferrite assembly and the permanent magnets are enclosed by a rectangular box-shaped yoke.
In this non-reciprocal circuit element, however, because the circuit substrate has a rectangular shape with longitudinal sides and lateral sides, the circuit substrate can be easily bent in the longitudinal direction. In addition, since lateral sides of the permanent magnets and the ferrite are disposed along the longitudinal sides of the circuit substrate, the permanent magnets and the ferrite do not function as a beam preventing bending of the circuit substrate in the longitudinal direction. Due to these reasons, the non-reciprocal circuit element is problematic in that if a mount board that has the circuit substrate mounted thereon is bent in the longitudinal direction due to an external force, the circuit substrate may break as a result of not having enough resistance to the bending force. The yoke has a box shape by bending four sides of a single magnetic plate. This is problematic in view of requiring a high cost of manufacture and in that a favorable direct-current magnetic circuit cannot be achieved because of gaps between edges of the four sidewalls of the yoke.
To prevent the circuit substrate from breaking, it is necessary to use a high-strength material for the substrate or to increase the thickness of the substrate. However, the material to be used is limited to a material most appropriate for achieving good electric properties for the substrate, and for this reason, the substrate must actually be designed to have a thickness that provides a required strength. This will lead to an increased height of the non-reciprocal circuit element.
Non-reciprocal circuit elements preferably satisfy the demands of compactness and low-profile structure, have as few variations in electric properties as possible, and have enhanced reliability.